<!DOCTYPE html>
<html>
    <head>
        <meta charset="utf-8">
        <title>simple</title>
        <style>
            .app{
                width: 800px;
                height: 600px;
                border: solid 2px red;
                position: relative;
            }
            .header{
                height: 64px;
                /* width: 100%; */
                background-color: burlywood;
                position: absolute;
                top: 0;
                left: 0;
                right: 0;
            }
            
        </style>
    </head>
    <body>
        <div class="app">
            <div class="header"></div>
            <div class="left-side"></div>
            <div class="main"></div>
            <div class="right-side"></div>
            <div class="footer"></div>
        </div>
    </body>
</html>